1. Field of the Invention
This invention pertains to silicon produced by silane pyrolysis in a fluidized bed reactor. More particularly, it pertains to an improved form of high purity silicon.
2. Description of the Prior Art
As a base material for semiconductor devices, silicon is more widely used than any other semiconductor. Silicon's dominant role results from its unique, favorable combination of semiconductor properties. Throughout the world, semiconductor grade silicon is produced by the Siemens process. In that process, slim rods of silicon are heated by electric current, and the heated rods are exposed in a suitable vessel to a gaseous mixture of hydrogen and trichlorosilane. Under the reaction conditions employed, silicon deposits on the rods, causing them to grow in size. In a modification of the Siemens process, silane is used as a gaseous source of silicon, rather than trichlorosilane.
Rods of polysilicon produced by the Siemens and modified Siemens processes described above, are not used directly. Instead, the rods are sawn or broken into chunks. The chunks are irregular in shape and about the size of a man's fist, or smaller. They are characterized by having irregular faces bounded by sharp, irregular surfaces. The chunks are not free flowing.
In order to make the solid state electronic devices used today, it is first necessary to transform polysilicon into monocrystalline silicon. Worldwide, about 80% of this basic material is produced by the Czochralski method; the rest mainly by the float zone method. In 1984, about 2500 metric tons of single crystal silicon was produced by the Czochralski process. This represents over 73 billion semiconductor devices.
In the Czochralski process, polysilicon is melted in a suitable crucible, a seed crystal is dipped into the melt, and then slowly withdrawn exactly vertically to the melt surface. Liquid silicon crystallizes on the seed. The result is an essentially single crystal rod with the angle between the cylindrical axis and the crystal orientation being close to zero.
The volume of the crucible employed cannot be totally filled by polysilicon chunks. Depending on the size of the chunks there is an unfilled crucible volume of about 30-50% between the chunks. To minimize the unfilled volume, chunks are piled above the top surface of the crucible, and to do this well, the chunks are stacked by hand. The operator visually selects the size and shape of the chunks in order to stack enough to charge the crucible with the proper weight of polysilicon. This process is laborious and time consuming. Furthermore, the operator's hands and gloves are frequently lacerated by the chunk edges.
In contrast to the chunks described above, the polysilicon of this invention is composed of free flowing, approximately spherical particles. They can be transported and handled readily. For example, they can be automatically charged to the melt crucible without being touched by the operator.
As more fully set forth below, the product of this invention is made by a fluidized bed process. At least one company makes polysilicon for its own internal requirements using a fluidized bed method in which trichlorosilane serves as the gaseous source of silicon. In contrast, the product of this invention is produced in a fluidized bed from silane. This reduces the opportunity or chances of chlorine contamination of the present product compared to the prior art material.
Use of silane in a fluidized bed reactor is not without problems. Silicon fines or dust is readily produced as a by-product. The dust is not only objectionable, but it represents an economical loss as well. Some aspects of dust formation are discussed in the art.
Eversteijn, Philips Res. Repts. 26, 134-144, (1971) comprises a study of gas phase decomposition of silane in a horizontal epitaxial reactor. It was found that gas phase decomposition is a serious factor that must be taken into account. In order to avoid gas phase decomposition, the maximum silane concentration in the hydrogen admitted to the reactor was 0.12-0.14 volume percent, depending on the gas temperature. When this critical silane concentration was exceeded, gas phase decomposition occurred giving rise to silicon fines which deposited on the substrate.
The Eversteijn article is referenced in Hsu et al, J. Electrochem Soc.: Solid State Science and Technology, Vol. 131, No. 3, pp. 660-663, (March, 1984). As stated there, the success of the Siemens process led to its universal adoption for producing semiconductor grade silicon, and the de-emphasis of fluidized bed process development. In 1975, the potential market for semiconductor grade silicon for photovoltaic use made fluidized bed (FB) production of polysilicon more attractive. Fluidized bed operation has the capabilities of high-throughput, continuous operation and low energy cost. Because silane has a low deposition temperature, and can be completely converted in a non-reversible reaction, it is attractive for use in fluidized bed (FB) operation. Additional advantages are the non-corrosive atmosphere, and ease of recycling by-product hydrogen. In conventional chemical vapor decomposition devices, there is a limit of silane concentration in hydrogen beyond which unwanted fines are homogeneously nucleated. Thus, in addition to the desired decomposition, silicon dust or fines appear in the gas phase. These particles vary in size from submicron to .about.10 microns, and present mechanical problems in the operation of the reactor. They are also difficult to transport. Dust and fines are considered losses in the process. Hence, conventional reactors are operated with low silane concentrations to prevent excess fines formation. In a fluidized bed reactor, less fines are generated because (i) there is less free space available for homogeneous nucleation and (ii) the silicon particles scavenge the fines and incorporate them into the deposition growth. Consequently, the net amount of fines is less than for chemical vapor deposition apparatus, and a fluidized bed reactor can be operated at much higher silane concentrations with greater throughput. Variables which effect the amount of fines elutriated were studied. Conclusions reached were as follows:
Elutriated fines increase with increased silane concentration, increased temperature, increased gas bubble size, and increased gas velocity. The authors selected 600.degree.-800.degree. C. and a gas velocity of U/U.sub.mf =3-8 as good operating parameters.
Another article, Hsu et al, Eighteenth IEEE Photovoltaic Specialists Conference (1984) pp. 553-557, discusses additional studies on fines formation. It states that silane pyrolysis in a fluidized bed reactor can be described by a six-path process: heterogeneous deposition, homogeneous decomposition, coalescence, coagulation, scavenging, and heterogeneous growth on fines. The article indicates that fines formation can be reduced by providing at a suitable bed location, a secondary source of silane for cementation.
The cited art clearly shows that production of silicon via decomposition of silane is complicated, and that provision of improved processes is not straight forward. Nonetheless, because of continuing advances in the electronics industry and the development of new products in that field, improvements in existing technology are needed to provide high purity silicon at reduced cost. This improved product of this invention arises from a process which enhances operation of fluidized bed methods, by providing means to make high quality product under high productivity operating conditions. The product is very pure, and has little dust. Its physical form is such that it is highly attractive to the silicon industry. For these reasons it is fair to say that the product is highly revolutionary and an advance in the art.